questions on how to make an object thicker

[Lucy Fan]

Hello. I'm making a dog sweater for my little chihuahua. I have the yarn picked out and it's 100% wool. I want the sweater to be really warm for my baby. I had made one previously for a friend's chihuahua and I stranded two colors of yarn which made it extra thick. I think I would like it extra thick like that for warmth with this one but I don't want to strand two colors of yarn. Is there a way to achieve the same effect by doing just a basic stockinette stitch?

Thanks in advance!